BSL is short for Breed-Specific Legislation. It is the banning of certain breeds because of their being perceived as a threat.
BSL is impractical, for several reasons. (1) It is how a dog is raised, trained, and socialized that shapes the greater part of it's temperament, not it's breed. To say that a dog is aggressive simply based on it's breed is nothing short of racism. (2) BSL does not work. People disobey the laws all the time, and more laws will not make a poodoo of difference to them. Dog fighting, for example, is against the law, yet some people still do it. (What DOES work is a dangerous dog ordinance, which does not target any specific breed.
For more reasons as to why BSL is a piece of bantha fodder, please see:
I am a proud member of ABLE (All Breeds Live Equally), a group of people fighting against BSL, and this group was started by Dogster members last year.
Here is our group on Dogster: